Current division for fault on high side of substation

[minus3db]

Hi all,
I was reading IEEE 80-2013 (Guide for Safety in AC Substation Grounding) and came across Figure 30 which shows current division during a substation fault.
Please see the below snapshot.
I am having difficulty in following KCL to arrive at the numbers shown in the diagram. I can see and understand that the returning current at the remote source is simply 1048A + 444A, but I don't follow how IG=742A at the faulted structure is determined. I feel really dumb as I thought that I understood the fundamentals of how fault current returns both via the neutral / shield wire and via ground, but I am not connecting the dots here. Would appreciate any insight that you folks could offer. Thank you.

 

[waross]

Delta contributions.
Consider three transformers in delta fed from phases A, B and C.
If you lose B phase, the transformer fed fro A-C still has full voltage applied.
The other two transformers are now in series across phases A and C.
This causes a voltage unbalance at the wye side of the distribution substation transformer.
The delta winding at the load substation tries to correct the unbalance,
The result of that is neutral/ground currents at the wye points of the wye windings.
Forgive my poor explanation,but consider the effect of two phase windings being in series across a healhty phase.

 

[bacon4life]

I share your confusion at various aspects of the diagram. The phase angles on the currents are not shown, which makes it hard to do KCL on portions where the labeled currents are out of phase.

 

[wcaseyharman]

Seems they are missing 36A somewhere. I would have thought the ground grid would carry 706A

 

[jghrist]

I think the missing 36A is because phase angles are not shown and the different currents do not have the same angles.

The diagram would be clearer if they showed the currents flowing into and out of the structure grounds.

• Neglecting phase angle errors, the current flowing from the structure grounds between the distribution and load substations is 448 - 99 = 349 into the remote source ground.
• The current flowing into shield wire between the distribution and remote source is 444 - 338 = 106. This current flows into the shield through the structure grounds. It originates from the distribution substation ground grid, the structure grounds between the distribution and load substations, and the load substation ground grid.